Spar

ABSTRACT

The invention relates to floating offshore facilities, columnar in form, often referred to as spars comprising a body having an upper column, a main compartment and a lower leg joined together to define the spar, wherein the spar further comprises an articulation for connecting an end of the lower leg to the seabed, wherein the lower leg geometry has been configured as having a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents. The invention also relates to methods for varying the dynamic response to wind, wave and forces applied by sea currents of the spar and to methods for transferring materials and personnel between a production facility such as the spar and at least one transportation apparatus such as a dynamically positioned vessel.

TECHNICAL FIELD

The present invention relates to methods and apparatus for transferring personnel and materials between a normally unmanned offshore facility which has some freedom to move on surface, and moving transportation apparatus by means of a dynamic gangway.

The invention has been devised particularly, although not necessarily solely, in relation to the method for transferring personnel and materials between vessels and offshore facilities having some freedom of movement on surface.

BACKGROUND ART

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

Thousands of gangway transfers of personnel and material, occur daily in the offshore oil, gas and windfarm industries, between dynamically positioned vessels and fixed offshore structures. A dynamically positioned vessel being a vessel which maintains position by an array of directional thrusters, and specialised position referencing devices.

Such operations are not attempted in all sea states. The generally accepted limit is a significant sea of 2.5 m or less.

However, if the offshore facility is not fixed, in that it is a floating buoy or spar tethered or otherwise fixed to the seafloor, complications arise.

Positioning and station keeping by a dynamically positioned vessel, near to an offshore structure, is controlled by close-in navigation systems such as fan beam and USBL (Ultra Short Base Line) acoustic referencing, using the structure, as reference.

If the facility has some freedom of movement, as occurs with a buoy or spar, the real time determination of the relative positions of the facility and vessel, and the ability to keep the relative positions synchronise within the physical and mechanical limits of gangway, becomes complex.

Further, the facility and the vessel will generally not have co-ordinated dynamic response, for different sea states.

This invention is specific to an offshore facility which is a columnar form structure (or spar), continuous from above the sea surface, to the seafloor or foundation feature near the seafloor, at which point the connection is an articulated joint, allowing sway.

Such a columnar structure (or spar), is omni-directional and subject to only two of the six forms of free body motion is a marine setting (refer diagram below FIG. 1). The two active motion forms are sway and yawl. Even if the bottom articulated joint is non rotating, analysis has shown the system generally has sufficient elasticity to generate some yawl.

In the case of such a columnar form structure (or spar), its metocean dynamic response is a function of its dimensions (and consequent hydrodynamic drag), the location and points of action of its mass and buoyancy, and the water depth.

The essence of the design is to discover the combinations of spar dimension and weight, which produce a near steady state spar condition for the desired operating conditions under which the vessel is employed, to deploy the gangway.

Mooring or lashing a vessel to a spar, creates additional load cases, for both the spar and the vessel, which under many conditions is not possible, and or will lead to failure of moorings, lashing or related appurtenances.

It is against this background that the present invention has been developed.

SUMMARY OF INVENTION

According to a first aspect of the invention there is provided a columnar structural (or spar) comprising a body having an upper column, a main compartment and a lower leg joined together to define the spar, wherein the spar further comprises an articulation for connecting an end of the lower leg to the seabed, wherein the lower leg geometry has been configured as having a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents (metocean conditions).

Preferably, the particular dynamic response of the spar is varied to achieve near steady state behaviour for specific sea states.

Preferably, the particular dynamic response of the spar is varied for the spar to be sufficiently steady, to function as a reference object for station keeping devices normally employed for close-in positioning of dynamically positioned vessels.

Preferably, the particular dynamic response of the spar is varied so that the motion of the spar is compatible with the motion of a dynamically positioned vessel thereby enabling the deployment of a gangway between the vessel and the spar, and the use of that gangway to transfer of personnel and materials to or from the vessel or spar.

Preferably, the particular dynamic response of the spar is varied to achieve accelerations within acceptable ranges to mitigate free surface effects in liquid containing process equipment located within the spar.

Preferably, the spar is adapted to provide a facility for a plug and play connection of flow lines and cables from the spar to subsea features served by the spar.

If the spar motions require further modification, catenary chain moorings from a location below the sea surface on the spar to the seabed, may be applied.

According to a second aspect of the invention there is provided a spar which is continuous surface to seabed, and with articulation connection at the seabed, has dynamic responses to metocean forces, which may be varied by manipulation of the lower leg geometry, in combination with residual buoyancy, for any particular upper body geometry, in any specific water depth.

Preferably, the dynamic response of the spar is varied to achieve near steady state behaviour for specific sea states.

Preferably, the dynamic response of the spar may be varied to be sufficiently stable, to function as a reference object for station keeping devices normally employed for close-in positioning of dynamically positioned vessels.

Preferably, the dynamic response of the spar is varied to create a motion form at surface, compatible with the motion form of a dynamically positioned vessel thereby enabling the deployment of a gangway between the two, and the use of that gangway to transfer of personnel and materials to or from each.

Preferably, the dynamic response of the spar may be varied to achieve accelerations within acceptable ranges to mitigate of free surface effects, in liquid containing process equipment, located within the spar.

Preferably, the spar further comprises an articulation for connection at the seabed, the spar being adapted for a single plug and play connection of flow lines and cables from spar to subsea features the spar serves.

Preferably, the dynamic response of the spar may be further modified by the addition of catenary cable or chain moorings from columnar body to the seafloor, if required.

According to a third aspect of the invention there is provided a method for transferring materials and personnel between a production facility and at least one transportation apparatus, the method for transferring materials and personnel comprising the steps of:

-   -   adjusting the movement of the production facility in such a         manner that the such that its movement on surface can be         replicated by the transportation apparatus     -   joining the production facility and transportation apparatus by         a dynamic gangway to allow transfer of materials and personnel;         and     -   conducting the transfer.

Preferably, the production facility comprises a spar in accordance with any one of the first or second aspect to the invention and at least one transportation apparatus comprises a dynamically positioned vessel.

Preferably, the steps of adjusting the movement of the production facility such as the spar comprises the steps of configuring the lower leg geometry to have a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents that permits the production facility to move in synchronism with the movement of the transportation apparatus.

According to a fourth aspect of the invention there is provided a method for varying the dynamic response to wind, wave and forces applied by sea currents of a spar comprising a body having an upper column, a main compartment and a lower leg joined together to define the spar, wherein the spar further comprises an articulation for connecting an end of the lower leg to the seabed, wherein the method comprises the step of configuring the lower leg geometry to a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of the excursion envelopes of a vessel engaged to a spar;

FIG. 2 is a schematic representation of the motion forms of a floating body such as a vessel;

FIGS. 3, 4 and 5 are schematic views of particular spars attached to the seafloor via different type of mooring means;

FIG. 6 show a particular arrangement of the spar shown in FIG. 5 including a catenary chain mooring, extending from the columnar body to seafloor, providing an additional means of moderating and controlling the surface motion;

FIGS. 7 to 24 shows graphs obtained through computer modelling for a particular arrangement of a spar shown in FIG. 5 subject to particular sea conditions;

FIG. 25 is a schematic view of a particular arrangement of a spar in accordance with an embodiment of the invention;

FIG. 26 is a schematic view of the main chamber of the spar of FIG. 25 depicting inner decks defining the interior of the main chamber;

FIG. 27 is a schematic view of a lower column of the spar of FIG. 25 depicting the shafts defining the interior of the lower column; and

FIG. 28 summarises the compartment capacities and contents for two conditions: towing and when deployed onto the storage tank for the particular arrangement of the spar shown in FIGS. 25 to 27.

DESCRIPTION OF EMBODIMENTS

FIG. 1 depicts typical excursion envelopes of a spar 10 and a dynamically positioned vessel, in close proximity. Item 1 and 2 on FIG. 1 are the loci of the contact points of a gangway, which might be placed between the two.

A gangway is so placed, has mechanical retract and extend limits, which typically might be 20 meter fully extended and 12 meter fully retracted. Therefore distance between points 1-2 has to be within these limits.

FIG. 2 depicts the motion forms on a floating body, whether it be a spar 10 or a transportation apparatus such as a vessel, and the six names of the six motions experienced by a floating body.

FIGS. 3, 4 and 5 depict typical spars 10 of different types, having reduced member size in the wave zone 3, to reduce wave impact, enlarged compartment below the wave zone 4, to create internal space for equipment and machinery normally required to a duty in the oil, gas or windfarm industries, which have to be maintained by transferring personnel and materials by gangway to and from the spar 10.

Each spar 10 form depicted in FIGS. 3 to 5 is fixed to the seafloor by different means, with difference motion response outcomes to wind, wave and current.

FIG. 3 depicts a free floating spar 10 on slack mooring 5, FIG. 4 depicts a forced immersion spar 10 fastened to the seafloor via tensioned tethers 6. Both spar 10 forms are subject to all six motions depicted in FIG. 2.

A spar 10 continuous surface to seafloor depicted in FIG. 5, with a non-rotating articulated connection 8 to the seafloor, is only subject to two motions: surge and sway.

The spar 10 depicted in FIG. 5 comprises an upper column 3, a main compartment 4 and a lower leg 7; in the particular arrangement, the lower leg 7 may comprise two leg section 7 a and 7 b joined together at a stepped section. In alternative, arrangements the lower leg 7 may be a continuous section extending from the main compartment 4 to the articulated section 8.

A spar 10 continuous surface to seafloor and founded to the seafloor depicted in FIG. 5, is not only subject to less motions, than other spar 10 forms, analysis and computer modelling has proved those motions to be of lower amplitude, and longer period, due to the hydrodynamic drag of the lower leg 7 a and 7 b.

Analysis and computer modelling has demonstrated that this period and related amplitude may be manipulated, for a specific sea state, by means of varying the lower leg geometry 7 a and 7 b, in combination with varying the residual buoyancy.

The lower leg may be stepped, as shown in FIG. 5, or be uniform.

In particular arrangements, the dynamic response of a spar 10 as the one depicted in, for example, FIG. 5 may be further modified by the addition of catenary cable or chain moorings, from the spar 10 to the seafloor.

Recognising that the transfer of personnel and materials by gangway, is normally only attempted in limited sea conditions of 2.5 m or less; the objective of this manipulation may be limited those conditions.

Analysis and computer modelling has shown that in certain instances, a near steady state can be achieved, and if desirable, accelerations modified. This steady state is usually involving some pitch, or lay-over of the spar 10, typically 1°-3°.

This near steady state, allows transfer of personnel and materials by gangway form a dynamically positioned vessel, to be similar to transfers to fixed offshore platforms.

This near steady state, allows the spar 10 to be a more stable reference for close in positioning systems, on a dynamically positioned vessel, manoeuvring beside.

In the offshore oil and gas industry, if a spar 10 is to function as a production facility, the liquid containing production separators, are subject to free surface effects, which impact process performance. The inherently lower range of motions, make the spar 10 of the type depicted in FIG. 5, more suitable to be employed as a production facility, relative to other forms.

A spar 10 employed in the offshore oil, gas and windfarm industries generally requires multiple connections flow line and cable such as subsea wellheads. Other spar 10 forms as depicted in FIGS. 3 and 4, these are separate bundles, requiring separate connection to the seafloor, and independent of the structural foundation connection.

In the case of spar 10 continuous surface to seafloor, non-rotating articulated joints are already available, which allow “plug and play” of these flow lines and cables, as part of the structural connection installation.

Plug and play is term with origins in the computer industry, referring to ability to connect to a device to a computer, and the computer automatically recognises the device. In the offshore industry plug and play generally means “connect ready to operate”.

Furthermore, FIG. 6 shows a particular arrangement of the spar 10 shown in FIG. 5 having mooring means 9 extending from the body of the spar 10 to the seafloor.

The inclusion of the mooring means is particular advantageous because it permits varying the dynamic response of the spar 10. The mooring means 9 may comprise catenary cable 9 or chain moorings 9 from the spar 10 to the seafloor; in a particular arrangement the catenary cable or chain moorings 9 may be attached to the main compartment of the spar 10.

Further, there is also provided a method for varying the dynamic response to wind, wave and forces applied by sea currents of the spar 10. The method comprises the step of configuring geometry of the lower leg to a particular geometry that in combination with a particular residual buoyancy of the spar 10, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar 10 provides a particular dynamic response to wind, wave and forces applied by sea currents.

Moreover, there is also provided a method for transferring materials and personnel between a production facility such has the spar 10 and at least one transportation apparatus such as a dynamically positioned vessel, the method for transferring materials and personnel comprising the steps of:

-   -   adjusting the movement of the production facility in such a         manner that the transportation apparatus moves in near         synchronous movement of the production facility;     -   joining the production facility and transportation apparatus to         allow transfer of materials and personnel; and     -   conducting the transfer.

The steps of adjusting the movement of the production facility such as the spar 10 comprises the steps of configuring the lower leg geometry to have a particular geometry that in combination with a particular residual buoyancy of the spar 10, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar 10 provides a particular dynamic response to wind, wave and forces applied by sea currents that permits the movement of the production facility in such a manner that the transportation apparatus moves in synchronism with the movement of the production facility.

FIGS. 7 to 24 contain extracts from the computer modelling performed. Some of the content is not relevant to the content and claims contained herein. Figures such as FIGS. 10, 12, 13 and 18 represent the input wave sets. Figures such as FIG. 14 to 16 represent the spar 10 response in terms of tilt angle and stability (or lack of stability) as a function of reserve buoyancy (termed tension in the analysis. The body of work performed is far more extensive than contained in FIGS. 7 to 24.

We refer now to FIGS. 25 to 28.

FIGS. 25 to 27 show a particular arrangement of the spar 10 in accordance with the embodiment shown in FIG. 5. FIG. 26 summarises the compartment capacities and contents for two conditions: towing and when deployed onto the storage tank for the particular arrangement of the spar 10 shown in FIG. 25

The spar 10 comprises an upper column 34, a main compartment 36 and a lower column 38. The lower column 38 comprises a connector 40 for attaching the spar 10 to the support surface such as a storage tank.

The upper column has a length of between about 16.25 to 17.02 meters; the main compartment has a length of between about 24.02 to 24.25 meters and the lower column has a length of between about 51 to 52.02 meters

The upper column 34 has an outer diameter of about 3.5 meters. The upper column 34 pierces out of the water surface while the spar 10 is moored to the support surface. The upper column 34 provides access to the interior of the spar 10. A conduit is contained in the lower column 34 that pierces out of the water surface to provide air and ballast ventilation, engine exhaust, flaring, and communications.

We refer now to FIG. 26. FIG. 26 depicts a schematic view of the main compartment 36.

The main compartment 36 comprises an upper section 42, a center section 44 and a lower section 46. The upper and lower section 42 and 46 have a conical configuration with their ends (having the larger diameter) being attached to the centre section 34 of the main compartment 36. The centre section 44 comprises an outer doubled wall shell that has an outer diameter and inner diameter. In this manner, a cavity 64 surrounding the main chamber 36 is defined. This cavity 64 protects against impact and impedes leakage of, for example, hydrocarbons into the sea. The cavity 64 also provides ballast space for ballasting of the buoy 32 during transportation, installation and operation.

In a particular arrangement of the buoy 32, the inner diameter is about 10 meters. The outer diameter is about 12 meters. The main compartment 36 comprises a plurality of first decks 56, 58 and 60.

A first pair of the first decks 56 and 58 is incorporated in the upper section 42. The deck 56 is mounted on the deck 58. The deck 56 comprises the control systems. The deck 58 comprises the power generation and ventilation systems.

The centre section 44 comprises a plurality of second decks 60. The decks 60 are stacked on the top of each other. The decks 60 are surrounded by the outer wall 62 defining the cavity 64 around the decks 60. The cavity 64 may comprise a plurality of compartments. The compartments are adapted to receive ballast. Further, the outer wall may be a double wall so as to impede leakage of hydrocarbons into the sea.

The lower section 46 of the main compartment 36 comprises a cone 66. The cone 66 is adapted to receive ballast.

The lower column 38 comprises three shafts 48 to 52 and a cone 54. In this particular arrangement, the total length of shafts 48 to 52 is about 46 meters.

The cone 54 of the lower column 38 has a length of about 5 meters. The cone 54 is adapted to receive the connector 40 for attaching the spar 10 to the support surface such as a storage tank.

As mentioned above the lower column 38 comprises three shafts 48 to 52 and a cone 54. The shafts 48 to 52 are stacked on the top of each other defining an upper shaft 48, a centre shaft 50 and a lower shaft 52. In a particular arrangement of the spar 10, the lower shaft 52 is partially filled with ballast such as ore ballast. The cone 54 is filled with ballast such as ore ballast.

The spar 10 comprises ballast compartments. These ballast compartments are selectively filled (or at least partially filled) with ballast for handling and transportation of the spar 10.

In a particular arrangement, the spar 10 comprises the following ballast compartments. A first ballast compartment comprises the cavity 64. As mentioned earlier this cavity 64 is defined by the inner and outer diameter of the main compartment 36. This first ballast compartment is filled with water during towing of the spar 10 but emptied when the spar 10 is deployed onto the support surface. This first ballast compartment is divided into three levels.

A second ballast compartment is the cone 66. The cone 66 defines the transition between the main compartment 36 and the lower column 38. This second ballast compartment is flooded during installation of the spar 10.

The third ballast compartment is the lower column 38. The shafts 48, 50 and 52 are generally flooded during upending of the buoy and during deployment of the spar 10 onto the support surface.

The shaft 52 is partially filled with iron ore ballast after installation. The cone 54 is permanently filed with iron ore and it is filled with the iron ore ballast at the fabrication yard prior to towing and deployment of the spar 10.

Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers 

1. A spar comprising a body having an upper column, a main compartment and a lower leg joined together to define the spar, wherein the spar further comprises an articulation for connecting an end of the lower leg to the seabed, wherein the lower leg geometry has been configured as having a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents.
 2. The spar in accordance with claim 1, wherein the particular dynamic response of the spar is varied to achieve near steady state behaviour for specific sea states.
 3. The spar in accordance with claim 1 or 2, wherein the particular dynamic response of the spar is varied for the spar to be sufficiently steady, to function as a reference object for station keeping devices normally employed for close-in positioning of dynamically positioned vessels.
 4. The spar in accordance with claim 1, wherein the particular dynamic response of the spar is varied so that the motion of the spar is compatible with the motion of a dynamically positioned vessel thereby enabling the deployment of a gangway between the vessel and the spar, and the use of that gangway to transfer of personnel and materials to or from the vessel or spar.
 5. The spar in accordance with any one of the preceding claims, wherein the particular dynamic response of the spar is varied to achieve accelerations within acceptable ranges to mitigate free surface effects in liquid containing process equipment located within the spar.
 6. A spar in accordance with any one of claims 1 to 5 wherein the spar is adapted to provide a facility for a plug and play connection of flow lines and cables from the spar to subsea features served by the spar.
 7. A spar in accordance with any one of the preceding claims further comprising moorings means extending from a location above the lower leg to the seabed for further modifying the dynamic response of the spar.
 8. A spar which is continuous surface to seabed, and with articulation connection at the seabed, has dynamic responses to wind, wave and current forces, which may be varied by manipulation of the lower leg geometry, in combination with residual buoyancy, for any particular upper body geometry, in any specific water depth.
 9. A spar in accordance with claim 8, wherein the dynamic response of the spar is varied to achieve near steady state behaviour for specific sea states.
 10. A spar in accordance with claim 8 wherein the dynamic response of the spar is varied to be sufficiently steady, to function as a reference object for station keeping devices normally employed for close-in positioning of dynamically positioned vessels.
 11. A spar in accordance claim 8 wherein the dynamic response of the spar is varied to create a motion form at surface, compatible with the motion form of a dynamically positioned vessel thereby enabling the deployment of a gangway between the two, and the use of that gangway to transfer of personnel and materials to or from each.
 12. A spar in accordance any one of claims 8 to 11 wherein the dynamic response of the spar may be varied to achieve accelerations within acceptable ranges to mitigate of free surface effects, in liquid containing process equipment, located within the spar.
 13. A spar in accordance with any one of claims 8 to 12 having an articulation for connection at the seabed, the spar being adapted for a single plug and play connection of flow lines and cables from spar to subsea features the spar serves.
 14. A spar in accordance with any one of claims 8 to 13 wherein the dynamic response of the spar may be further modified by the addition of catenary cable or chain moorings from the buoy to the seafloor.
 15. A method for transferring materials and personnel between a production facility and at least one transportation apparatus, the method for transferring materials and personnel comprising the steps of: adjusting the movement of the production facility in such a manner that the transportation apparatus moves in synchronism with the movement of the production facility; joining the production facility and transportation apparatus to allow transfer of materials and personnel; and conducting the transfer; wherein the step of adjusting the movement of the production facility comprises incorporating the production facility onto a spar as described in any of the claims 1 to
 14. 16. A method in accordance with claim 15 wherein the production facility comprises at least one transportation apparatus comprises a dynamically positioned vessel.
 17. A method in accordance with claim 15 or 16 wherein the steps of adjusting the movement of the production facility such as the spar comprises the steps of configuring the lower leg geometry of the spar to have a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents that permits the production facility to move in synchronism with the movement of the transportation apparatus.
 18. A method for varying the dynamic response to wind, wave and forces applied by sea currents of a spar comprising a body having an upper column, a main compartment and a lower leg joined together to define the spar, wherein the spar further comprises an articulation for connecting an end of the lower leg to the seabed, wherein the method comprises the step of configuring the lower leg geometry to a particular geometry that in combination with a particular residual buoyancy of the spar, and for any particular geometry of the upper column and main compartment and in any specific water depth, the spar provides a particular dynamic response to wind, wave and forces applied by sea currents. 